1. Field of the Invention
This invention relates to the field of fluorescent lighting and more particularly is directed to a fluorescent lamp ballast characterized by superior load current regulation such that one ballast can run lamp loads of a wide wattage range while maintaining approximately constant current through the lamps.
2. State of the Prior Art
Fluorescent lamps are low pressure mercury arc discharge devices. A filament at each end of a sealed lamp tube is heated by a filament current, and a glow discharge is sustained between the electrodes by a sufficiently high lamp voltage applied across the lamp tube. Such a fluorescent lamp behaves as a negative impedance in that, as current increases through the lamp its resistance drops. Unless externally limited the current will rise until the lamp is destroyed. A ballast is designed to supply the necessary lamp and filament voltages while limiting the current to the lamp at a level which provides optimum light output without damaging the lamp tube. A ballast is essentially a voltage source which feeds a current limiting reactive component in series with the lamp load. One type of voltage source used for this purpose is an LC resonant tank circuit, while the current limiting component is the internal leakage inductance of a transformer used to convert the resonant tank voltage to the higher lamp voltage and lower filament voltages.
One specially demanding ballast application is in fluorescent lighting of aircraft cabins, such as in passenger jetliners. Aircraft lighting systems must meet requirements of low weight, small size and a minimum of electromagnetic interference (EMI). Small size and light weight is achieved by minimizing the number of components used, and specially by minimizing the number and size of the magnetic components. Some ballasts achieve the objective of low EMI by operating at low AC frequencies, such as 60 cycle or 400 cycle AC. Inductors and capacitors required by low frequency ballasts tend to be bulky and heavy, however. High frequency operation, e.g. 20 to 30 Kilohertz, alleviates this shortcoming. High frequency ballasts are more prone to generate EMI however, and the circuit must be designed to minimize this problem.
Existing high frequency ballasts are designed to regulate the output current (thus regulating the lamp light output) for a particular fluorescent lamp load, i.e. one or more lamps of a given total wattage. If used with loads of greater or lesser wattage, the current through the lamp is substantially greater or lesser than optimal for that particular load. Too large a current shortens lamp life, while insufficient current may cause the lamp to flicker.
What is needed, particularly for aircraft applications, is a universal ballast capable of regulating load current for a range of fluorescent lamp loads, for example from 18 Watts to 4 Watts, or even from 36 Watts to 4 Watts, while holding the load current approximately constant. Such a universal ballast should have a minimum of components and particularly a minimum of heavy inductors or bulky capacitors to minimize size and weight. The current and voltage delivered to the lamp should be sinusoidal in waveshape to minimize EMI.